Influenza virus continues to threaten humans and remains a major health concern around the world. Following influenza virus infection, the host produces type I interferon (IFN) to inhibit viral spread. Although type I IFN interferes with virus replication and stimulates host immunity to protect the host from harmful viral cytotoxicity, the IFN itself can cause a detrimental inflammatory response. Therefore, factors that regulate the local induction and activity of type I IFN must be identified and traced to better understand and manipulate the interplay between the host and the influenza virus. The sphingolipids are bioactive lipid mediators, which include sphingosine 1-phosphate (S1P) and ceramide, that regulate multiple cellular conditions with important therapeutic potential. However, the action mode by which sphingolipid metabolism modulates the host protective signaling and immune response against influenza virus infection remains unknown. Preliminary data indicate that overexpression of S1P lyase renders cells resistant to influenza virus infection and viral cytopathic effects. Activation of JAK/STAT type I IFN signaling is critical for the host defensive mechanism mediated by S1P lyase. In contrast, cells overexpressing sphingosine kinase (SK) 1 are more susceptible to the infection, and an inhibitor blocking SK1 displayed anti-influenza viral activity. Further, a ceramide analog dramatically enhanced the induction of type I IFN in dendritic cells (DCs) upon influenza virus infection and also enhanced DC maturation and T cell stimulation. These results indicate the capacity of sphingolipid metabolism to control host protection and immunity in part via the function of type I IFN. In this study, the regulation of influenza virus propagation and viral pathogenesis by S1P-metabolizing enzymes and ceramide will be further investigated. The unique research aims and experiments include 1) determining the intracellular signaling mechanism by which S1P-metabolizing enzymes control influenza virus replication, 2) defining the role of SK1 blockade in influenza pathogenesis and host immunity to the infection by using SK1- specific inhibitors, and 3) investigating the mechanism of ceramide's effect on influenza virus spread and host immune responses, specifically via antigen-presenting DCs and anti-viral T cells. Ultimately, the research proposed here should produce a detailed understanding of cellular signals that regulate viral replication and promote the development of therapeutic interventions to remedy viral diseases.